1. Field of the Invention
The present invention relates to a configuration of an oil pump that can achieve a size reduction of the entire pump, reduction in wear of the rotor during operation and that can also achieve longer pump life and reduction in production cost.
2. Description of the Related Art
There are, as conventionally known, internal gear oil pumps with a relief valve. Japanese Patent Application Laid-open No. S63-246482 discloses a specific configuration of such an oil pump (see FIGS. 8A and 8B). The pump according to Japanese Patent Application Laid-open No. S63-246482 has in general a configuration, in which a circular recess 6 in which inner and outer rotors are arranged has a smooth cover attachment surface 22 therearound to attach a cover 24, and a plurality of bolt holes 23 drilled at suitable locations for fastening the cover 24.
An oil return passage 26 is formed in the cover attachment surface 22 in the form of a groove from near a discharge chamber 11 toward an inlet chamber 10. One end of this oil return passage 26 opens to an inlet passage 12, while the other end extends as far as to a portion adjacent the discharge chamber 11. The cover attachment surface 22 is thus divided into a pump chamber-side portion 22a that surrounds the circular recess 6, and an outer portion 22b. 
A side hole 27a, which is drilled in a middle position of a relief passage 27 that opens to an outlet passage 14, opens to the oil return passage 26. A known relief valve 28 is mounted in the relief passage 27, so that lubricating oil under excess pressure is discharged into the oil return passage 26 through the side hole 27a to flow back to the inlet chamber 10 when the pressure of discharged oil exceeds a predetermined value.